fix const correctness in Diagonal::coeffRef (fix found by failtests)
fix const correctness in Diagonal::coeffRef (fix found by failtests)

Zefz/eigen,TSC21/Eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,ritsu1228/eigen,Zefz/eigen,toastedcrumpets/eigen,pasuka/eigen,toastedcrumpets/eigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,TSC21/Eigen,Zefz/eigen,pasuka/eigen,Zefz/eigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,toastedcrumpets/eigen,ritsu1228/eigen,pasuka/eigen,toastedcrumpets/eigen,ritsu1228/eigen,pasuka/eigen